The invention relates to an exhaust gas line for an internal-combustion engine which, in the cold state, is subjected to prestress which is eliminated at normal engine operating temperature.
An exhaust gas line of this general type is disclosed in German Patent Document DE-PS 36 35 478. There, the exhaust gas line comprises several sections which are each composed of two groove-shaped parts which, resting on one another by means of flanges molded to both longitudinal edges, form a line cross-section between one another. During mounting, a radial prestress is generated by means of expanding which, when the normal operating temperature is reached, is compensated for as a result of thermal expansion. In the axial direction, gaps are situated between the individual sections so that relative movements between sections that occur in the longitudinal direction because of thermal expansion, are not impaired. At the normal operating temperature, the gaps are almost closed so that only a little exhaust gas can leak out into the space between the interior line and the gastight liquid-cooled housing surrounding the interior line. A disadvantage of this type of construction consists in the high expenditures in manufacturing and the mounting of the exhaust gas line. In addition a large installation space is required.
An exhaust gas line according to the German Patent Document DE-PS 37 43 851 is constructed as a one-piece pipe section which is used for guiding hot exhaust gases from an exhaust pipe to an exhaust gas turbine. In order to allow for axial expansion, the fastening of the exhaust gas line on the surrounding water-cooled housing takes place by means of screws which are distributed in a cross-sectional plane. In the cold condition, the exhaust gas line has a smaller radial dimension than the diameter on which the fastening points are situated. As a result, the exhaust gas line experiences a radial deformation during the cold mounting. The deformation of the exhaust gas line, which is a result of this chucking, is compensated by the thermal expansion at the normal operating temperature, and the exhaust gas line is therefore virtually free of tension at the normal operating temperature. This type of an exhaust gas line naturally has only a limited overall length. When exhaust gas lines of a larger length are required, the exhaust gas line must be constructed of several individual pipe sections which are lined up with one another. However, this results in high expenditures for manufacturing and mounting.
European Patent Document EP-PA 0 185 934 shows a flexible hose wound from metal strips which may also be used in exhaust gas systems for internal-combustion engines. In such a hose, the individual windings can also be constructed to be axially displaceable with respect to one another so that thermal expansions in the axial direction are permitted. However, the patent does not state how such an exhaust gas line can be disposed without any tension at high exhaust gas temperatures.
It is an object of the invention to provide a one-piece exhaust gas line of any arbitrary length, which can be manufactured at low cost, is easy to mount, and which is essentially free of tension at normal operating temperatures.
This object can be achieved by having an internal-combustion engine exhaust gas line enclosed by a gastight water-cooled housing to which it is fastened at several points in such a manner that in a cold state, the exhaust gas line is subjected to a radial prestress which is just eliminated at normal engine operating temperature. The exhaust gas line is made of a helically wound metal strip. In order to avoid tensions as a result of an axial thermal expansion of the exhaust gas line, the exhaust gas line is fastened to the surrounding water-cooled housing in such a manner that the edges of the metal strip in the cold state of the exhaust gas line are aligned to form a helical gap therebetween. In order to avoid leaking of exhaust gases into the space between the interior exhaust gas line and the exterior housing, the gap is just closed at normal engine operating temperature due to thermal expansion of the strip. In order to prevent tensions as a result of a radial expansion of the exhaust gas line, the exhaust gas line is also fastened in such a manner that it is radially prestressed in the cold state. However, because of thermal expansion, the radial deformation resulting from the prestress at the cold temperature is compensated so that at normal engine operating temperatures, the exhaust ga line is essentially free of tension.
So that the prestress (created during a cold mounting) can be compensated, the gap between the edges of the metal strip is precisely maintained. A method for determining the position of mounting bores in the strip can be precisely determined irrespective of manufacturing tolerances by: (1) forming a flat metal strip; (2) helically coiling the strip so that side edges of the strip are spaced from one another to form a helical gap; (3) screwingly inserting a coil spring into the gap so as to fill the gap with the spring; (4) feeding the coiled strip and spring into an internal cylindrical cavity in a drilling jig that has cut-outs for determining the location of the bores and which has an internal diameter which is less than the internal diameter of the exhaust line when in an expanded shape due to thermal expansion caused by exhaust gas of the internal-combustion engine; (5) applying an axial force to one end of the coiled strip to force another end of the coiled strip against a bottom of the cylindrical cavity and cause the coil strip to expand to said internal diameter of the cylindrical cavity so that the location for the bores in the exhaust line are identified at the cut-outs in the drilling jig.